How you add them to an article differs depending on whether you use VisualEditor or source mode.

VisualEditor

In VisualEditor, select the Template option from the Insert menu.

In the menu type "Infobox". You'll see several different types of infoboxes, but the one you want is infobox film. (If you want, please feel free to test out the different infoboxes in your sandbox!)

Click the add template button. This will bring up a variety of different fields that you can add to the infobox. If you select the option to display more fields, you will see other options. (Be careful of this with some infoboxes, as some can have dozens of options.)

Select the options that you want to go into the infobox. Since this will be for a film, you will want to select the film's common name (ie, the movie title), director, and so on - everything that you have the information for. Once you've selected the items this box will look like this:

Enter in the information that you have and then press the insert button. You'll end up with an infobox like this:

With the infobox in place, all you have to do is save the page when you're done!

Source code

Inserting an infobox with source code can be a bit trickier, since you have to know the exact name of the infobox so you can find it. If you're uncertain about the name, try checking a page for a similar topic - that can often point you in the right direction, however I highly recommend adding the infobox in VisualEditor and/or asking your Content Expert for help.

In source mode, copy and paste the infobox's code into the article or draft.

Unlike the VisualEditor mode, where you had to add the fields one by one, this adds them all. Like with VisualEditor, add in the information that you have and save the article. Don't worry if you have an empty field - it won't show up when you save unless it's populated.

As for what goes into the infobox, it should be treated like anything you add to Wikipedia - it should be something that can be verified with a reliable source.